ESTRO 2021 Abstract Book

S1673

ESTRO 2021

Zero patients had a re-scan and re-plan as a result of the Day 0 appointment; however, 3 abdominal patients were rescanned after fraction one. As a result of Day 0, 18 patients required additional imaging off protocol; 10 patients received CBCT with an additional 4DCBCT on fraction 1, 7 had 4DCBCT for all fractions, 1 patient had CBCT and 4DCBCT all fractions. Nine of these 18 patients were having abdominal treatments i.e. with complex motion management techniques including EEBH or abdominal compression. The remaining 9 were lung metastasis patients. There were 137 imaging annotations by the COs and 171 by the RTTs. There was agreement in 54.9% of the imaging annotations, there was acceptable variation in 40.5% and unacceptable variation in 4.6%, with no statistically significant findings. The measurement of agreement was limited as the evaluation indicated there was wide variation amongst the COs and/or the RTTs with regards to the criteria reviewed offline for Day 0. Thus, the most common criteria reviewed were collated and discussed among the MDT. It was agreed for SABR patients the following criteria should be utilised by both COs and RTTs when reviewing the imaging, Table 1.

Conclusion The results show there has been very little impact from Day 0 on treatment for SABR patients. Thus, Day 0 has been removed for bone, node, spine, and lung. However, Day 0 has not been removed for abdominal cases as rescans were required in these patients, and their motion management technique is complex. In terms of RTT role in Day 0, it appears that RTTs are making appropriate offline decisions; however, overall as a centre, it was unexpectedly found that there was variability in the criteria used to review SABR imaging. Thus, consistency in the SABR offline review and training will be introduced before the service can be managed completely by RTTs. PO-1966 Intrafractional organ motion in cervical carcinoma brachyradiotherapy : Dosimetric impacts P. Marolt 1 , R. Hudej 2 , B. Šegedin 3 , H.B. Zobec Logar 4 , M. Kobav 3 1 Institute of Oncology Ljubljana, Departments of Radiophysics, Ljubljana, Slovenia; 2 Institute of Oncology Ljubljana, Department of Radiophysics, Ljubljana, Slovenia; 3 Institute of Oncology Ljubljana, Department for Brachyradiotherapy, Ljubljana, Slovenia; 4 Onstitute of Oncology Ljubljana, Department for Brachyradiotherapy, Ljubljana, Slovenia Purpose or Objective During brachytherapy treatment organs in the pelvic region can move due to the peristaltic or variations in bladder and rectum filling, resulting in differences between planned and absorbed doses to organs at risk. In this study we evaluated these differences by comparing dose volume histogram parameters D2cm3 and D0.1cm3. The purpose of this study was to evaluate impact of the intrafractional organ motion on absorbed dose in organs at risk in cervical carcinoma during pulse 15 patients with locally advanced cervical cancer, treated with pulse dose rate brachytheapy after external beam radiotherapy and chemotherapy were enrolled in the study. Clinicaly approved treatment plans and post irradiation plans were compared . All plans were magnetic resonance imaging - based. Post irradiation magnetic resonance imaging was done after pulse dose rate irradiation was completed. Clinically approved and post irradiation magnetic resonance images for each patient were registered with respect to the applicator. We compared organ volumes, D2cm3 and D0.1cm3 parameters for bladder, rectum, sigmoid and small intestine in both plans. Results For all patients differences in bladder and rectum volumes were found, but the differences were not statistically significant. D2cm3 for the rectum was higher in the control plan (p = 0,038). No statistically significant differences were found in D2cm3 and D0.1cm3 for other organs at risk. However, large variations in D2cm3 were observed for individual patients, and should be taken into account. Similar results were reported in literature by other authors. Conclusion The results show that organ at risk volumes change during pulse dose rate brachytherapy treatment, which results in dose variations. Although the average differences in observed dose volume histogram prameters were small, there were some individual large variations in D2cm3 which should be taken into account, especially when in clinicaly approved treatment dose rate brachytherapy. Materials and Methods